There are many ways to write a good handwritten birthday message in a card and examples include "I hope that you had a wonderful day today and were able to eat your favorite foods and do all of your favorite things." Another example might include a lengthier message with specific memories written about for the birthday recipient to reminisce over.
There is not a "right" or "wrong" way to create a personalized message for someone on their birthday. A person who chooses to write a sentence or two can send an equally thoughtful message as someone who chooses to write several paragraphs. Usually whena person has spent time choosing a card that has the perfect words, it is more appropriate to write a shorter phrase because the card's words already express the person's feelings. For more generic cards, a lengthier message may be the best way to express the perfectbirthday wish.
The difference in the length of the message as well as how many details are shared will also depend upon the relationship. Many employees like to give their bosses birthday cards, but will only provide a short handwritten birthday wish such as "It has been a pleasure to work with you and I want to wish you a happy birthday." These birthday wishes will need to be more concise and kept professional.